Significance The goal of this project is to document individual differences in behavioral, neuroendocrine, and immunological characteristics of rhesus macaques. We expect that, as in humans, such differences may have health consequences. Objectives In the past budget year, we examined the stability of personality assessments made on our 18 subject animals (adult male rhesus monkeys) in 1993. Animals were observed while interacting with each other in stable social dyads. In addition, we examined individual differences in cortisol and immunological responses to pharmacological manipulations. Blood samples were drawn at 5 time points over a two-hour period during three conditions saline, ketamine, or telazol pretreatment. Results We demonstrated considerable behavioral stability in our personality dimensions, and found personality correlates to early physiological events following SIV inoculation. During our pharmacological challenge, we found significant effects of personality on our measures. In this study, animals judged high in 'Excitability' (i.e. above the median on this trait) had significantly lower plasma cortisol concentrations and significantly higher CD4+ cell numbers at all time points. Moreover, whereas animals low in 'Excitability' showed an increase in their CD4/CD8 ratio over the two hour period, animals high in 'Excitability' exhibited a higher ratio that did not change over time. Future Directions These data demonstrate that there are important differences between individuals on dimensions reflecting personality that persist over time and are reflected in behavioral and physiological measures. The existence of such individual differences has implications for research design. Knowledge of such characteristics may permit investigators to select subjects for research projects that are more homogeneous. By reducing the potential variation in outcome measures, adequate statistical power can be maintained while the number of subjects is reduced. Our future research is aimed at further exploring trait-like physiological characteristics using this, as well as a larger sample. KEY WORDS personality, temperament, immune system, social behavior, cortisol FUNDING NIH Grant RR00169